1. Field of the Invention
The present invention relates to a Fresnel lens sheet and a rear projection screen employed for a rear projection television.
2. Description of the Prior Art
In a rear projection television being one of a large-screen televisions, an image light projected from a light source reflects on a reflection mirror forms an image over a screen. The screen consists basically of a Fresnel lens sheet, a lenticular lens sheet and a front panel respectively from the light source side resulting in three pieces constitution.
A rear projection screen employed for a rear projection television is usually obtained by assembling a Fresnel lens sheet and a lenticular lens sheet. However, a light reflection on the surface of the Fresnel lens sheet and a labyrinth light inside the Fresnel lens sheet cause some distortions on the projected image. For example, in the case where a screen of the rear projection television is watched up from down side in a short distance, the projected image on the upper area of the screen is observed a little down side of it being magnified. The phenomenon that these multiple images are observed is generally called as ghost. The reason why the ghost appears is considered that the labyrinth light within the Fresnel lens sheet and the reflected light from the surface of the light source side of the Fresnel lens sheet reflect back on a reflection mirror, and then enter into the Fresnel lens sheet again. Further, the ghost has a tendency of being emphasized because, in late years, the luminance of the light source is elevated for improving an image luminance.
This phenomenon will be explained with the use of a drawing (FIG. 1). In a rear projection screen obtained by deploying reflection mirror 2 on a rear of Fresnel lens sheet 1, incident light 3 from a light source enters into Fresnel lens sheet 1 and emit to an observer's side. On the surface 5 of the light source side of the Fresnel lens sheet 1, the light partly reflects and reflects again on reflection mirror 2 thereby emitting as ghost light 4b to the observer's side. Further, incident light 3 into Fresnel lens 1 partly reflects on the surface 6 of the outgoing radiation side of the Fresnel lens 1 and reflects again on the surface 5 of the light source side of the Fresnel lens sheet 1 and on reflection mirror 2 thereby emitting as ghost lights 4a and 4c. 
In order to reduce such a ghost, Japanese Patent Application Laid-Open No. Hei 5-158153 proposes a formation of hairlines in the direction perpendicular to longitudinal direction of a lenticular lens over the surface of the other side of a Fresnel lens forming surface. Further, Japanese Patent Application Laid-Open No. Hei 5-127257 proposes a formation of a uniform rugged structure over the surface of the other side of a Fresnel lens forming surface and also proposes specifying the haze value, half brightness angle, ⅓ brightness angle, etc., within a certain range. Furthermore, Japanese Registered Patent No. 3056571 discloses a formation of an antireflection film over the surface of the other side of a Fresnel lens forming surface.
As described in the foregoing prior art, diffusing or decreasing the labyrinth lights within the screen and surface reflection by means of forming a rugged structure or antireflection process over the surface of light source side of the Fresnel lens sheet reduces the ghost. However, the effects of the foregoing prior art are insufficient for the rear projection televisions employing the recent light sources with the highly elevated luminance.